Zinc isotope fractionation during mantle melting and constraints on the Zn isotope composition of Earth's upper mantle

The zinc (Zn) stable isotope system has great potential for tracing planetary formation and differentiation processes due to its chalcophile, lithophile and moderately volatile character. As an initial approach, the terrestrial mantle, and by inference, the bulk silicate Earth (BSE), have previously been suggested to have an average delta Zn-66 value of similar to+0.28% (relative to JMC 3-0749L) primarily based on oceanic basalts. Nevertheless, data for mantle peridotites are relatively scarce and it remains unclear whether Zn isotopes are fractionated during mantle melting. To address this issue, we report high-precision (+/- 0.04%; 2SD) Zn isotope data for well-characterized peridotites (n = 47) from cratonic and orogenic settings, as well as their mineral separates. Basalts including mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) were also measured to avoid inter-laboratory bias. The MORB analyzed have homogeneous delta Zn-66 values of + 0.28 +/- 0.03% (here and throughout the text, errors are given as 2SD), similar to those of OIB obtained in this study and in the literature (+0.31 +/- 0.09%). Excluding the metasomatized peridotites that exhibit a wide delta Zn-66 range of -0.44% to + 0.42%, the non-metasomatized peridotites have relatively uniform delta Zn-66 value of + 0.18 +/- 0.06%, which is lighter than both MORB and OIB. This difference suggests a small but detectable Zn isotope fractionation (similar to 0.1%) during mantle partial melting. The magnitude of inter-mineral fractionation between olivine and pyroxene is, on average, close to zero, but spinels are always isotopically heavier than coexisting olivines (Delta Zn-66(Spl-Ol) = + 0.12 +/- 0.07%) due to the stiffer Zn-O bonds in spinel than silicate minerals (Ol, Opx and Cpx). Zinc concentrations in spinels are 11-88 times higher than those in silicate minerals, and our modelling suggests that spinel consumption during mantle melting plays a key role in generating high Zn concentrations and heavy Zn isotopic compositions of MORB. Therefore, preferential melting of spinel in the peridotites may account for the Zn isotopic difference between spinel peridotites and basalts. By contrast, the absence of Zn isotope fractionation between silicate minerals suggests that Zn isotopes are not significantly fractionated during partial melting of spinel-free garnet-facies mantle. If the studied nonmetasomatized peridotites represent the refractory upper mantle, mass balance calculation shows that the depleted MORB mantle (DMM) has a delta Zn-66 value of + 0.20 +/- 0.05%(2SD), which is lighter than the primitive upper mantle (PUM) estimated in previous studies (+ 0.28 +/- 0.05%, 2SD, Chen et al., 2013b; + 0.30 +/- 0.07%, 2SD, Doucet et al., 2016). This indicates that the Earth's upper mantle has a heterogeneous Zn isotopic composition vertically, which is probably due to shallow mantle melting processes. (C) 2016 Elsevier Ltd. All rights reserved.